Abstract

Joshua tree (Yucca brevifolia Engelm.) is a large, arborescent member of the yucca genus. It is an endemic and visually dominant plant in portions of the Mojave Desert, USA. We document the unique and heretofore unreported directional orientation of its flower panicles. The flower panicles grow primarily at the tips of branches that are oriented to the south. When branches with flower panicles are not oriented in a southerly direction, the flower panicles themselves tend to bend or tilt toward the south. This strategy maximizes exposure of the panicles to direct solar radiation, which, within the latitudes where the Joshua tree grows, is always from the south. Such a strategy may minimize the energetic cost of translocating photosynthates from the plant’s leaf rosettes to the flowers. The flower panicles create large, light-colored landing pads for the obligate nocturnal moth pollinator. Residual warmth in the flower panicles may provide a thermal reward for the moth pollinator that emerges shortly after sunset.